In particle physics, a generation is a division of the elementary particles. Particle physics is a branch of Physics that studies the elementary constituents of Matter and Radiation, and the interactions between them In Particle physics, an elementary particle or fundamental particle is a particle not known to have substructure that is it is not known to be made Between generations, particles differ only by their mass. Mass is a fundamental concept in Physics, roughly corresponding to the Intuitive idea of how much Matter there is in an object All interactions and quantum numbers are identical. In Physics, a fundamental interaction or fundamental force is a mechanism by which particles interact with each other and which cannot be explained in terms Quantum numbers describe values of conserved numbers in the dynamics of the Quantum system. There are three generations according to the Standard Model of particle physics. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles
Each generation is divided into two leptons and two quarks. Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. The two leptons may be divided into one with electric charge −1 (electron-like) and one neutral (neutrino); the two quarks may be divided into one with charge −1/3 (down-type) and one with charge +2/3 (up-type). Electric charge is a fundamental conserved property of some Subatomic particles which determines their Electromagnetic interaction.
|First generation||Second generation||Third generation|
|Neutrino||Electron neutrino||Muon neutrino||Tau neutrino|
|Down-type quark||Down quark||Strange quark||Bottom quark|
|Up-type quark||Up quark||Charm quark||Top quark|
Each member of a higher generation has greater mass than the corresponding particle of the previous generation. Leptons are a family of fundamental Subatomic particles comprising the Electron, the Muon, and the Tauon (or tau particle as well as their The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J The muon (from the letter mu (μ--used to represent it is an Elementary particle with negative Electric charge and a spin of 1/2 The tau lepton (often called the tau, tau particle, or occasionally the tauon; symbol) is a negatively charged Elementary particle with Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost Neutrinos are Elementary particles that travel close to the Speed of light, lack an Electric charge, are able to pass through ordinary matter almost In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. The down quark is a first-generation Quark with a charge of -(1/3 e. The strange quark is a second- generation Quark with a charge of &minus(1/3 e and a strangeness of &minus1 The bottom quark is a third-generation Quark with a charge of − e. In Physics, a quark (kwɔrk kwɑːk or kwɑːrk is a type of Subatomic particle. The up quark is a particle described by the Standard Model theory of Physics. The charm Quark is a second-generation quark with an electric charge of +(2/3 e. The top quark is the third- generation up-type Quark with a charge of +(2/3 e. For example: the first-generation electron has a mass of only 0. 511 MeV/c2, the second-generation muon has a mass of 106 MeV/c2, and the third-generation tau lepton has a mass of 1777 MeV/c2 (almost twice as heavy as a proton).
All ordinary atoms are made of particles from the first generation. History See also Atomic theory, Atomism The concept that matter is composed of discrete units and cannot be divided into arbitrarily tiny Electrons surround a nucleus made of protons and neutrons, which contain up and down quarks. The nucleus of an Atom is the very dense region consisting of Nucleons ( Protons and Neutrons, at the center of an atom The proton ( Greek πρῶτον / proton "first" is a Subatomic particle with an Electric charge of one positive This article is a discussion of neutrons in general For the specific case of a neutron found outside the nucleus see Free neutron. The second and third generations of charged particles do not occur in normal matter and are only seen in extremely high-energy environments. Neutrinos of all generations stream throughout the universe but rarely interact with normal matter.
Within the Standard Model, fourth and further generations have been ruled out by theoretical considerations. The Standard Model of Particle physics is a theory that describes three of the four known Fundamental interactions together with the Elementary particles Some of the arguments against the possibility of a fourth generation are based on the subtle modifications of precision electroweak observables that extra generations would induce; such modifications are strongly disfavored by measurements. Furthermore, a fourth generation with a light neutrino (one with a mass less than about 40 GeV/c2) has been ruled out by measurements of the widths of the Z boson (LEP, CERN). The W and Z bosons are the Elementary particles that mediate the Weak force. The European Organization for Nuclear Research (Organisation Européenne pour la Recherche Nucléaire known as CERN Nonetheless, searches at high-energy colliders for particles from a fourth generation continue, but as yet no evidence has been observed. In such searches, fourth-generation particles are denoted by the same symbols as third-generation ones with an added prime (e. g. b′ and t′). Given the unlikeliness of any such particles being discovered, no other names have been seriously proposed.
There is some debate as to whether the muon and tau particles are actually fundamental in a strict sense, or whether they are better described as excited states of an electron.  Richard Feynman is quoted in Genius, the life and science of Richard Feynman as questioning the fundamentality of all of the leptons (including the electron), explaining:
Physicists have yet to reconcile the notion of the higher generation leptons as excited states of the electron with the theory that the electron is a point charge. A point charge is an idealized model of a particle which has an Electric charge. However, other theories as to the nature of an electron include the possibility that an electron is a charged conducting surface, with a surface tension to prevent it from flying apart under the repulsive forces of the charge. 
It is hoped that a comprehensive understanding of the relationship between the generations of the leptons may eventually explain the ratio of masses of the fundamental particles, and shed further light on the nature of mass generally, from a quantum perspective.